Wiki · Concept
Thermal power and electrical power
MWt is the heat the core produces; MWe is the electricity that reaches the grid. Two thirds are lost between one and the other, and that is physics, not bad engineering.
A reactor is characterized by two powers. Thermal power, in megawatts thermal or MWt, is the heat generated by fissions in the core. Electrical power, in MWe, is what the turbine and generator convert into electricity. Atucha II, for example, produces 2,160 MWt and delivers 693 MWe net.
The ratio between the two is the thermal efficiency, which depends above all on the steam temperature. A PWR with steam at 285 °C is around 33%; an AGR with steam at 540 °C reaches 41%. The rest of the heat goes to the river, the sea or the cooling tower, as in any thermal plant. It is the second law of thermodynamics, not a design flaw.
Within electrical power there is a distinction between gross and net. Gross is what the generator produces; net subtracts what the plant itself consumes in pumps and auxiliary systems, between 3 and 7%. This atlas always uses net MWe, which is what reaches the grid, and flags values that are estimated.
Quick facts
| Typical efficiency | PWR and BWR 32 to 34%; CANDU 30 to 31%; AGR 41%; fast reactors and HTGR 40 to 43% |
|---|---|
| House load | 3 to 7% of gross power |
| Load factor | actual energy divided by the maximum possible in the year; the best reactors exceed 90% |
| Rule of thumb | MWe ≈ MWt × 0.33 for light water |